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Full text of "Handbook Of Chemical Engineering - I"

CRYSTALLIZATION                                   403
copper-sulphate crystals, so that strips of lead were hung in the tanks on which the la/rgest crystals deposited (see remarks concerning "rock candy," p. 404), and the a,-t-fcempt was made to get as great a crop as possible on these strips.
The material that formed at the bottom and lower portions of the sides of the tank Was known as ''bottom crystal." The amount of this inferior material was increased t>y the "windows," although interest on capital was saved by the shortened time of Crystallization.
On the other hand, the time of crystallization was increased, and the relative amount °f bottom crystals diminished, by carefully floating about an inch of water on top of "fclie sulphate mother liquor just after dropping the liquor into the tanks. The water Had a less radiating power (or else a lower coefficient of conduction) than the mother liquor, and materially slowed down the drop of temperature in the early hours of the crystallizing process.
In this connection it may be noted that trade prejudices must be. considered in crystallizing. Some ferrous-sulphate (copperas) crystals that were far purer than the trade had been used to were unsalable because the trade (at that time) was used to a crystal with some brownish stains and these were all a beautiful green! Happily "fclme cured, their too brilliant appearance.
Crystallization in Motion.—The highest development of the crystallization ixi motion process is in sugar factories. Because the juices of sugar-producing p>lants contain many substances other than sugar, only a part of which can be commercially removed, the production of sugar of the purity to which we are accustomed, 99.7 to 99.9 per cent, is necessarily a complicated matter.
Before the introduction of the "crystallization in motion" process cane-sugar crystallization was carried out in three stages. The juice, properly " clarified" by heating with milk of lime followed by settling and separation of the clear juice from the sludge, is concentrated in multiple-effect evaporators to ** thick sirup " or "meladura," in which form it is taken into the vacuum pan for final concentration. By careful adjustments of heat and vacuum the formation of crystals is brought about. When the crystals have grown to the desired size "bite "massecuite" is discharged into iron "sugar wagons" in which it stands until cold, when it is dximped into mixers and when of uniform consistency is run by gravity to the centrifugal machines where the molasses is expelled. The crystals are "first sugar" of 94 to 96 per cent purity which goes upon the market as raw sugar for refining. The molasses is "first molasses" and is reconcentrated in vacuum pan, cooled and centrifuged as above. The crystals are "second sugar" and are melted in fresh juice and recovered as "first sugar." The molasses is ** second molasses." It cannot be boiled to "grain" or so as to form crystals, but is boiled to a consistency called "string proof" and stored in sugar wagons for from 2 to 6 months, usually until just before the beginning of the next cam-IP aign when it is centrifuged and the "third sugar7' worked up with fresh juice. The "third molasses" is the final cane molasses and is of 27 per cent to 33 per cent "purity," which is the per cent of sugar in the total solids present.
In the most modern process the "thick juice" is boiled to grain as above described, t>\it the rn assecuite instead of being discharged into cars is run into huge "crystallizers " S ft. in diameter by 30 ft. long, lying horizontal, and provided with powerful stirring arms on a horizontal shaft. These crystallizers are usually provided either with a water jacket or with cooling coils for controlling the temperature. The separation of -the sugar from the viscous mass (massecuite) of crystals and molasses is a very different problem from crystallization from relatively pure water solutions as in the cases